After fabrication, a mask or reticle ("mask") should be inspected before use to make sure that the mask accurately defines the desired pattern. This is because any defects in the mask pattern will be transferred to the substrate (e.g., wafer) during use of the mask in microlithography. It is also desirable to perform an inspection of a wafer at suitable moments during manufacture of devices on the wafer. As used herein, a "sample" for inspection purposes can be a wafer, mask, reticle, or analogous structure having a pattern that is the subject of the inspection.
Conventionally, a type of scanning electron microscope (SEM) has been used to inspect samples to detect pattern defects. The surface of the sample is scanned using a single finely drawn electron beam. Impingement of the beam on the sample generates secondary electrons. A pattern defect at a location on the sample is detected by comparing an intensity signal of the secondary electrons to, for example, a reference signal corresponding to the same location on the pattern.
The conventional SEM as summarized above exhibits high resolution compared to an optical microscope. However, because only one very narrow electron beam is used for scanning, a long time is required to scan the entire surface of the sample. This undesirably results in low throughput. If the scanning speed of the electron beam is increased to reduce the scanning time per sample, the signal-to-noise ratio (S/N ratio) of the secondary electron signal sensed by the detector can be too small for reliable detection of errors on the sample.
Many masks define patterns made up of identical portions repeated many times at a fixed pitch over the entire pattern. Each identical portion can be scanned relative to a reference signal. However, because the mask is conventionally scanned by one electron beam, the periodicity of the pattern defined by the mask cannot be exploited and throughput is not improved.